1. Field of the Invention
The invention relates to the field of structure-borne noise reduction, and more particularly to apparatus for countering vibrations of elements coupled to the structure.
2. Description of the Prior Art
Ring laser gyroscopes (RLG) utilize two monochromatic laser beams propagating in opposite directions about a closed loop. Rotation of the apparatus about the loop axis effectively increases the beam path length in one direction and decreases the beam path in the opposite direction. Since the laser frequencies of the two counter-rotating beams are functions of the lasing path length, the differential path length established by the rotation of the RLG causes a frequency difference between the two beams. The magnitude and sign of this frequency difference are representative of the RLG""s rate and direction of rotation and may be monitored for these purposes in manners well known in the art. At low rotation rates, the frequency difference between the counter-rotating beams is small and the beams tend to resonate at the same frequency, i.e. lock-in, and the RLG appears to be stationary. This lock-in prevents the RLG from sensing rotation rates that are at or below the lock-in rate. To reduce the lock-in rate, the RLG is mechanically oscillated, dithered, about its axis to establish rotation in one direction and then the other. Such dithering provides a signal at the output terminals that is substantially independent of the mechanical oscillation while maintaining an apparent rotation in each direction, thus reducing the lock-in rotation rate.
The dithering causes the structure on which the RLG is mounted to vibrate, thereby generating structure-borne noise which adversely effects equipment mechanically coupled to the mounting structure. One method of the prior art for reducing structure-borne noise is disclosed in U.S. Pat. No. 5,012,174 issued to Charles M. Adkins, et al and assigned to the assignee of the present invention. Adkins, et al teach a device which is attached directly to the RLG platform and electronically establishes counter vibrations of the platform to cancel vibrations induced by the dithering RLG. The apparatus taught by Adkins, et al, however, is complex mechanically and electrically and is too expensive for use with the relatively inexpensive RLG.
Another method of the prior art for reducing structure-borne noise is disclosed in U.S. Pat. No. 5,267,720 issued to James R. Brazell, et al and assigned to the assignee of the present invention. Brazell, et al teach the use of a pair of noise attenuator assemblies positioned along mutually perpendicular rotational axes. Each noise attenuator includes a precision ground valve spring captivated in a highly damped elastomeric material molded to a machined housing. Matching of the noise attenuators and alignment of the rotational axes must be performed to close tolerances to achieve the required platform stabilization. Suppression of mechanical resonances of the sensor supporting structure is achieved by applying a viscoelastic constrained layer to 90 percent of the external surfaces. To meet shock, vibration, and structure-borne noise isolation, high precision machining, tight tolerances on molded elastomers, matched preloaded noise attenuators, and extensive inspection are required. Thus, the device is difficult to manufacture and assemble and therefore, costly.
The above limitations were overcome by the invention disclosed in U.S. Pat. No. 6,056,259 issued to Jamil I. Lahham and assigned to the assignee of the present invention. This patent teaches the utilization of a tunable auxiliary mass coupled to an element vibrating at forced vibration frequencies. The auxiliary mass is tuned to vibrate at the forced vibration frequencies out of phase with the element vibrations, thus reducing the structure-borne noise.
Included in the auxiliary mass is a four cavity chamber, each having a flat base and a flat top. These cavities are filled to capacity with steel shots to provide the mass and stiffness required to achieve the desired tuned vibration frequency. Chamber and steel shot tolerances, however, establish a random steel shot arrangement geometry. This randomness, however slight, may, for some assemblies, provide an auxiliary mass that may not be appropriately tuned, thereby requiring iterations of re-assemblies and concomitant structure-borne noise measurements to achieve the desired auxiliary mass vibration frequency.
In accordance with the present invention the randomness of the steel shot arrangement is substantially eliminated by constructing the four cavity chamber with the base of each cavity containing a multiplicity of grooves which contain and guide spherical elements, such as steel ball bearings. The grooves are positioned and dimensioned so that the spherical elements fill each cavity in a touch relationship with adjacent spherical elements and cavity walls. Arranging the spherical elements and grooves in this manner maintains the spherical elements motionless in each cavity.
These and other aspects of the invention will be more fully understood by referring to following detailed description and the accompanying drawings.